Cyclic nucleotide phosphodiesterases (PDEs) show specificity for purine cyclic nucleotide substrates and catalyze cyclic AMP (cAMP) and cyclic GMP (cGMP) hydrolysis (Thompson, W. J. (1991) Pharma. Ther. 51:13-33). Cyclic nucleotide phosphodiesterases regulate the steady-state levels of cAMP and cGMP and modulate both the amplitude and duration of cyclic nucleotide signal. These cyclic nucleotides are important second messengers in many physiological processes, including regulation of vascular resistance, cardiac output, visceral motility, immune response, inflammation, neuroplasticity, vision, and reproduction (Hetman, J. M. (2000) Proc. Nat. Acad. Sci. 97: 472-476). At least ten different but homologous PDE gene families are currently known to exist in mammalian tissues (Sasaki, T. et al. (2000) Biochem. Biophys. Res. Comm. 271(3):575-583). Most families contain distinct genes, many of which are expressed in different tissues as functionally unique alternative splice variants. (Beavo (1995) Physiological Reviews 75:725-748 and U.S. Pat. No. 5,798,246).
All cyclic nucleotide phosphodiesterases contain a core of about 270 conserved amino acids in the COOH-terminal half of the protein thought to be the catalytic domain of the enzyme. A conserved motif of the sequence HDXXHXX has been identified in the catalytic domain of all cyclic nucleotide phosphodiesterases isolated to date. The cyclic nucleotide phosphodiesterases within each family display about 65% amino acid homology and the similarity drops to less than 40% when compared between different families with most of the similarity occurring in the catalytic domains.
Most cyclic nucleotide phosphodiesterase genes have more than one alternatively spliced mRNA transcribed from them and in many cases the alternative splicing appears to be highly tissue specific, providing a mechanism for selective expression of different cyclic nucleotide phosphodiesterases (Beavo supra). Cell-type-specific expression suggests that the different isozymes are likely to have different cell-type-specific properties.
Type 1 cyclic nucleotide phosphodiesterases are Ca2+/calmodulin dependent, are reported to contain three different genes, each of which appears to have at least two different splice variants, and have been found in the lung, heart and brain. Some of the calmodulin-dependent phosphodiesterases are regulated in vitro by phosphorylation/dephosphorylation events. The effect of phosphorylation is to decrease the affinity of the enzyme for calmodulin, which decreases phosphodiesterase activity, thereby increasing the steady state level of cAMP. Type 2 cyclic nucleotide phosphodiesterases are cGMP stimulated, are localized in the brain and are thought to mediate the effects of cAMP on catecholamine secretion. Type 3 cyclic nucleotide phosphodiesterases are cGMP-inhibited, have a high specificity for cAMP as a substrate, and are one of the major phosphodiesterase isozymes present in vascular smooth muscle and play a role in cardiac function. One isozyme of type 3 is regulated by one or more insulin-dependent kinases.
Type 4 cyclic nucleotide phosphodiesterases are the predominant isoenzyme in most inflammatory cells, with some of the members being activated by cAMP-dependent phosphorylation. Type 5 cyclic nucleotide phosphodiesterases have traditionally been thought of as regulators of cGMP function but may also affect cAMP function. High levels of type 5 cyclic nucleotide phosphodiesterases are found in most smooth muscle preparations, platelets and kidney. Type 6 cyclic nucleotide phosphodiesterase family members play a role in vision and are regulated by light and cGMP.
PDE7A2, a Type 7 cyclic nucleotide phosphodiesterase family member, is found in high concentrations in skeletal muscle. Work using mouse tissue has shown that PDE7A2 is found in high concentrations in skeletal muscle, followed by spleen. Lower levels were found in brain, heart, kidney, lung, and uterus (Han, P. et al. (1997) J. Biol. Chem. 272:16152-16157). A member of the type 7 cyclic nucleotide phosphodiesterases identified as PDE7B has been cloned (Hetman, supra). In the mouse, PDE7B has been found in high concentrations in pancreas followed by brain, heart, skeletal muscle, eye, thyroid, ovary, testis, submaxillary gland, epididymus, and liver. PDE7B has been identified as a cAMP-specific PDE (Hetman, supra). A human PDE7B cDNA was cloned by Sasaki et al. and a dot blot analysis was made to determine the expression pattern of PDE7B in human tissues. Human PDE7B transcripts were particularly abundant in the putamen and caudate nucleus. Sasaki et al. reported the effects of various PDE inhibitors on recombinant human PDE7B. The human PDE7B gene is thought to be localized at chromosome 6q23-24. The EPM2A gene, which is related to progressive myoclonus epilepsy, is located at 6q24, making it possible that PDE7B and its gene is linked to epilepsy (Sasaki et al., supra). Gardner et al. ((2000) Biochem. Biophys. Res. Comm. 272:186-192) also identified and characterized human PDE7B. Gardner et al. reported that mRNA for human PDE7B was most highly expressed in caudate nucleus, putamen, and occipital lobe of the brain, heart, liver, ovary, pituitary gland, kidney, small intestine, and thymus. A phylogenetic alignment of the 230 amino acid catalytic domain of PDE7B (amino acids 172-420) with representatives of other PDEs showed that PDE7B has the highest homology to and clusters with PDE7A (70% identity). Gardner et al. also studied the effects of a variety of standard PDE inhibitors on PDE7B.
A listing of cyclic nucleotide phosphodiesterase families 1-7, their localization and physiological role is given in Beavo supra. A Type 8 family is reported in U.S. Pat. No. 5,798,246.
Many functions of the immune and inflammatory responses are inhibited by agents that increase intracellular levels of cAMP (Verghese (1995) Mol. Pharmacol. 47:1164-1171) while the metabolism of cGMP is involved in smooth muscle, lung and brain cell function (Thompson W. (1991) Pharma. Ther. 51:13-33). A variety of diseases have been attributed to increased cyclic nucleotide phosphodiesterase activity which results in decreased levels of cyclic nucleotides. For example, one form of diabetes insipidus in the mouse has been associated with increased phosphodiesterase Family 4 activity and an increase in low-Km cAMP phosphodiesterase activity has been reported in leukocytes of atopic patients. Defects in cyclic nucleotide phosphodiesterases have also been associated with retinal disease. Retinal degeneration in the rd mouse, human autosomal recessive retinitis pigmentosa, and rod/cone dysplasia 1 in Irish setter dogs have been attributed to mutations in the Family 6 phosphodiesterase, gene B. Family 3 phosphodiesterase has been associated with cardiac disease.
Many inhibitors of different cyclic nucleotide phosphodiesterases have been identified and some have undergone clinical evaluation. For example, Family 3 phosphodiesterase inhibitors are being developed as antithrombotic agents, as antihypertensive agents and as cardiotonic agents useful in the treatment of congestive heart failure. Rolipram, a Family 4 phosphodiesterase inhibitor, has been used in the treatment of depression and other inhibitors of Family 4 phosphodiesterase are undergoing evaluation as anti-inflammatory agents. Rolipram has also been shown to inhibit lipopolysaccharide (LPS) induced TNF-alpha which has been shown to enhance HIV-1 replication in vitro. Therefore, rolipram may inhibit HIV-1 replication (Angel et al. (1995) AIDS 9:1137-44). Additionally, based on its ability to suppress the production of TNF alpha and beta and interferon gamma, rolipram has been shown to be effective in the treatment of encephalomyelitis, the experimental animal model for multiple sclerosis (Sommer et al. (1995) Nat. Med. 1:244-248) and may be effective in the treatment of tardive dyskinesia (Sasaki et al. (1995) Eur. J. Pharmacol. 282:72-76).
There are also nonspecific phosphodiesterase inhibitors such as theophylline, used in the treatment of bronchial asthma and other respiratory diseases, and pentoxifylline, used in the treatment of intermittent claudication and diabetes-induced peripheral vascular disease. Theophylline is thought to act on airway smooth muscle function as well as in an anti-inflammatory or immunomodulatory capacity in the treatment of respiratory diseases (Banner et al. (1995) Eur. Respir. J 8:996-1000) where it is thought to act by inhibiting both cyclic nucleotide phosphodiesterase cAMP and cGMP hydrolysis (Banner et al. (1995) Monaldi Arch. Chest Dis. 50:286-292). Pentoxifylline, also known to block TNF-alpha production, may inhibit HIV-1 replication (Angel et al. supra). Thiopyrimidine derivatives substituted at position 2 of the pyrimidine ring have been taught as inhibitors of cGMP or thromboxane A2 (TXA2), which is known to induce platelet aggregation and to contract smooth muscle (U.S. Pat. No. 5,869,486). A list of cyclic nucleotide phosphodiesterase inhibitors is given in Beavo supra.
Cyclic nucleotide phosphodiesterases have also been reported to affect cellular proliferation of a variety of cell types and have been implicated in the treatment of various cancers. Bang et al. ((1994) Proc. Natl. Acad. Sci. USA 91:5330-5334) reported that the prostate carcinoma cell lines DU 145 and LNCaP were growth-inhibited by delivery of cAMP derivatives and phosphodiesterase inhibitors and observed a permanent conversion in phenotype from epithelial to neuronal morphology; Matousovic et al. ((1995) J. Clin. Invest. 96:401-410) suggest that cyclic nucleotide phosphodiesterase isozyme inhibitors have the potential to regulate mesangial cell proliferation; Joulain et al. ((1995) J. Mediat. Cell Signal 11:63-79) reports that cyclic nucleotide phosphodiesterase has been shown to be an important target involved in the control of lymphocyte proliferation; and Deonarain et al. ((1994) Brit. J. Cancer 70:786-94) suggest a tumor targeting approach to cancer treatment that involves intracellular delivery of phosphodiesterases to particular cellular compartments, resulting in cell death.
Accordingly, compounds that interact with cyclic nucleotide phosphodiesterases may provide treatments for various diseases and conditions caused by errors in regulation of cyclic nucleotide phosphodiesterase mediated processes. The present invention advances the state of the art by providing such compounds.